พลาสติก - ภาพรวม

Plastic materials display
properties that are unique when compared to other materials and have
contributed greatly to quality of our everyday life. Plastics, properly
applied, will perform functions at a cost that other materials cannot match.
Many natural plastics exist, such as shellac, rubber, asphalt, and cellulose ; however, it is man's ability to synthetically
create a broad range of materials demonstrating various useful properties that
have so enhanced our lives. Plastics are used in our clothing, housing,
automobiles, aircraft, packaging, electronics, signs, recreation items, and
medical implants to name but a few of their many applications.

The synthetic plastic industry started in 1909 with the development of a phenol
formaldehyde plastic (Bakelite) by Dr. L. H. Bakeland.
The phenolic materials are, even today, important
engineering plastics. The development of additional materials continued and the
industry really began to blossom in the late 1930's. The chemistry for nylons,
urethanes, and fluorocarbon plastics were developed; the production of
cellulose acetate, melamine, and styrene molding compounds began; and
production of commercial equipment to perform the molding and vacuum forming
processes began.

Acrylic sheet was widely used in aircraft windows and canopies during World War
II. A transparent polyester resin (CR-39), vinylidene
chloride film (Saran), polyethylene, and silicone resins were also developed.
The first polyethylene bottles and cellulose acetate toothpaste tubes were
manufactured during this time period.

The post war era saw the production of vinyl resins started, the use of vinyl
films, molded automotive acrylic taillights and back-lighted signs introduced,
and the first etched circuit boards developed. The injection molding process
entered commercial production. Due to the newness of the materials, the
properties and behavior of the plastic materials were not well understood. Many
products were introduced that failed, creating a negative impression about plastics
in the public's mind.

Chemists continued the development of materials, such as ABS, acetals, polyvinyl fluoride, ionomers,
and polycarbonate. The injection molding, thermoforming, extrusion, transfermolding, and casting processes were all improved.
This allowed the industry to provide an even greater number of cost-effective
products suitable for many, more demanding engineering applications.

In the early days...
Bakelite (phenolic resin) is most often actually
a product called Catalin. Both, along with Plaskon, are formaldehyde based plastics. Allow me to
expand (with liberal borrowing from Dr. Stephen Z. Fadem
- a true expert).

Around the turn of the century, the Belgian born scientist Dr. Leo Baekeland, working as an independent chemist, came upon the
compound quite by accident. He sold his rights to Velox
to Eastman Kodak for three quarters of a million dollars and started developing
a less flammable bowling alley floor shellac; bowling
was becoming the latest rage in New
York City. Dr. Baekeland
soon realized that a resin that was both insoluble and infusible could have a
much wider appeal when used as a molding compound. He obtained a patent and
started the Bakelite Corporation around 1910.

Phenolic resin could be produced in a multitude of
colors, commonly yellow, brown, butterscotch, green and red. Omitting the
pigment could produce a transparent or translucent effect. The resin could be
molded or cast, depending on variations in the formula. For molding, the
formula was cooked until resinous, spread out in thin sheets to harden, then
ground to a fine consistency. At this point, powdered fillers and pigment were
added, to enable the resin to be molded and to add color. This mixture was then
put through hot rollers which created large sheets of colored, hardened resin.
These sheets were then ground into a very fine powder which was molded under
high heat and pressure into the final product form. As a molded material the
resin's drawback was the limited range of colors which could be created. For
casting, the formula was modified slightly, enabling the resin to be poured
into lead molds and then cured in ovens until it polymerized into a hard
substance. The liquid resin could be tinted to any color or "marbleized"
by mixing two colors together.

For the first ten years or so after its introduction, the resin was used
primarily to make electrical and automobile insulators and heavy industrial
products. Eventually, uses for the resin spread into the consumer market.
Castings were made in the shape of cylinders or blocks, and then sold to
novelty and jewelry makers. Industrial designers began experimenting with the
new material. Fine craftsmen sculpted the molded products on fast wheels with
razor-like tools to carve out designs that the world has not seen since; after
World War II, most companies switched to creating designs through the use of
patterned molds, instead of hand-carving. Bakelite replaced flammable
celluloid, previously the most popular synthetic material for molded items, as
a major substance for jewelry production.

The process to the collector of today may not be significant, as Bakelite is
now treasured for its unique, unreproducible beauty.
A deeply carved half inch bangle bracelet may sell for $225.00, and a two and
one half inch bangle may command $900.00. Bakelite often acquires a patina
within a few months to a few years of its date of production, and metamorphisizes into a completely different appearing
color. The red, white and blue Bakelite designs of yesterday have mellowed into
lovely yellows, reds and blacks, enhancing further the value of those rare
pieces which have continued to maintain their original color and luster.

Bakelite's many uses allowed it to become a standard item in the family home of
the 1930s and 1940s. It was frequently found in the kitchen, in the form of
flatware handles, rabbit or chicken napkin holders, salt and pepper shakers, or
serving trays. During the Depression Bakelite sold more than any other
commercial product, and was loved by the public for its brilliant and cheerful
colors and its affordability.

When the Bakelite patent expired in 1927, it was acquired by the Catalin Corporation that same year. They began mass
production under the name "Catalin," using
the cast resin formula which enabled Catalin to add
15 new colors to the original five produced by the Bakelite Corporation, which
used the limited color range molded formula, as well as the now-famous marbleized
effect. One of their most notable products was the Fada
bullet radio. The Catalin Corporation was responsible
for nearly 70% of all phenolic resins that exist
today.

Bakelite-Catalin was sold mostly by Saks Fifth
Avenue, B. Altman and Bonwit Teller, but was also on
the shelves of F.W. Woolworth and Sears. To the wealthy socialites, whose
husbands had fallen on tough times during the Depression, with Tiffany diamonds
and Cartier jewelry now well beyond their means, the vibrantly colorful carved
jewelry adorned with rhinestones became de riguer for
cocktail parties and formal dinners. Yet, Catalin and
Bakelite were within everyone's reach with Depression prices ranging from
twenty cents to three dollars. Diana Vreeland, editor
of Vogue, often spoke of the versatility of Bakelite, as did Elsa Schiaparelli, who was constantly contracting with the
Bakelite and Catalin Corporations for exclusive
buttons for her dress designs.

But in 1942 Bakelite and Catalin suspended sales of
their colorful cylinders to costume jewelry manufacturers in order to
concentrate on the wartime needs of a nation which had totally shifted its
focus. Defense phones and aviator goggles, as well as thousands of other
Bakelite products, found their way to armed forces around the world. The scheme
shifted from the 200 vibrant colors which brightened the dark days of the
Depression to basic black, the no-nonsense symbol of a nation at war. By the
end of the war, new technology had given birth to injection-molded plastics,
and most manufacturers switched to less labor-intensive and more practical
means of developing products. The next generation of plastics had been born -
Acrylic, fiberglass, and vinyl - and they were molded into products commonplace
in our everyday lives today.

Occasionally plastics are still improperly used and draw negative comments. The
thousands of successful applications that contribute to the quality of our life
are seldom noticed and are taken for granted. Remember, MATERIALS DON'T FAIL,
DESIGNS DO.

The number of variations or formulations possible by combining the many
chemical elements is virtually endless. This variety also makes the job of
selecting the best material for a given application a challenge. The plastics
industry provides a dynamic and exciting opportunity.

Plastics encompass a large and varied group of materials consisting of
different combinations or formulations of carbon, oxygen, hydrogen, nitrogen
and other organic and inorganic elements. Most plastics are a solid in finished
form; however, at some stage of their existence, they are a liquid and may be
formed into various shapes. The forming is usually done through the
application, either singly or together, of heat and pressure. There are over
fifty different, unique families of plastics in commercial use today and each
family may have dozens of variations.

How are plastics made? The word "MER" is a Greek word that means
"part." This part of a plastic is a unique combination of molecules
and is called a "MONOMER." It is like a single link in a chain. The
monomers are then fused or joined together, usually using heat and pressure, to
make long chains that result in a material with a useful blend of properties. Using another Greek word "POLY" which means
"many", the long chain of "mers"
forms a "POLYMER." The monomers are held together in a polymer
chain by the strong attractive forces between molecules, while much weaker
forces hold the polymer chains together. The polymer chains can be constructed
in many ways. Some simplified examples of the way polymers are built are shown
in Figure 1:

HOMOPOLYMERS:
A-A-A-A-A-A-A-A-A-
(Polymers constructed from a single material)
Examples of polymers built this way are polyethylene, and some acetals.

Figure 1b

COPOLYMERS:
(Polymers constructed from two different materials)
ALTERNATING TYPES: A-B-A-B-A-B-A-B-A-B-
Some examples of alternating copolymers are ethylene-acrylic and ethylene-ethyl
acrylate.

Some
examples of grafted copolymers are styrene-butadiene, styrene-acrylonitrile, and some acetals.

Figure 1d

TERPOLYMERS:
A-B-C-A-B-C-A-B-C-
(Polymers constructed from three different materials)
An example of a terpolymer is acrylonitrile-butadiene-styrene
(ABS).

Figure 1e

The two monomers in a
copolymer are combined during the CHEMICAL REACTION of polymerization.
Materials called "ALLOYS" are manufactured by the SIMPLE MIXING of
two or more POLYMERS with a resulting blending of properties which are often
better than either individual material. There is no chemical reaction in this
process. Some examples of "alloys" are Polyphenylene
Oxide/High Impact Styrene, Polycarbonate/ABS, and ABS/PVC.

MOLECULAR
WEIGHT
It is important for the chemist to know how long the polymer chains are
in a material. Changing the length of the chains in a thermoplastic material
will change its final properties and how easily it can be shaped when it is
melted.

The "REPEATING UNIT" or molecular group in the homopolymer
(Figure 1) is A-, the group of molecules in the copolymer A-B-, and in the terpolymer A-B-C-. The number of repeating units in the
polymer chain is called the "DEGREE OF POLYMERIZATION." If the
repeating unit has a molecular weight (the combined weight of all of the
molecules in the repeating unit) of 60 and the chain or polymer has 1000 repeating
units, then the polymer has a "MOLECULAR WEIGHT" of 60 x 1000 =
60,000. The molecular weight is a way of measuring how long the polymer chains
are in a given material.

The molecular weight of plastics is usually between 10,000 and 1,000,000. It
becomes increasingly difficult to form or mold the plastic with the application
of heat and pressure as the molecular weight increases. A molecular weight of
about 200,000 is about the maximum for a polymer to still permit reasonable processability. Some higher molecular weight materials,
like Ultra High Molecular Weight Polyethylene (UHMWPE) which has a molecular
weight from 3,000,000 to 6,000,000, can be cast using processes specifically
designed to shape it.

CRYSTALLINE/AMORPHOUS
MATERIALS
Some of the polymers, because of their geometry, pack together very
tightly in a regular order when the material is hard and are called
"CRYSTALLINE." These polymers usually exhibit a very sharp melting
point; that is, they are solid. Then with a small increase in temperature they
become liquid or melt. An illustration of a sharp melting point is the melting
of ordinary candle wax. Some examples of crystalline plastic materials are
nylon, acetal, polyethylene, and polypropylene. The
crystalline polymers provide superior properties, but they tend to shrink a
considerable amount as they cool and reharden.

Materials that do not crystallize upon solidifying are called
"AMORPHOUS." These materials demonstrate a gradual softening as the
temperature is increased. Some examples of amorphous materials are acrylics,
polycarbonate, and ABS. These materials are usually not as easily processed as
the crystalline material since they do not flow as easily during molding.

Polymerchemists may also vary how the polyrnerchains are constructed by grafting as shown in
Figure 1d. This allows the properties of a material to be further tailored to
meet the specific needs of an application.

THERMOPLASTIC/THERMOSET
MATERIALS
The terms "THERMOSETTING" and "THERMOPLASTIC" have
been traditionally used to describe the different types of plastic materials. A
"THERMOSET" is like concrete. You only get one chance to liquify and shape it. These materials can be
"cured" or polymerized using heat and pressure or as with epoxies a
chemical reaction started by a chemical initiator.

A "THERMOPLASTIC", in general, is like wax; that is, you can melt it
and shape it several times. The "thermoplastic" materials are either
crystalline or amorphous. Advances in chemistry have made the distinction
between crystalline and amorphous less clear, since some materials like nylon
are formulated both as a crystalline material and as an amorphous material.

Again, the advances in chemistry make it possible for a chemist to construct a
material to be either thermoset or thermoplastic. The
main difference between the two classes of materials is whether the polymer
chains remain "LINEAR" and separate after molding (like spaghetti) or
whether they undergo a chemical change and form a three dimensional network
(like a net) by "CROSSLINKING." Generally a crosslinked
material is thermoset and cannot be reshaped. Due to
recent advances in polymer chemistry, the exceptions to this rule are
continually growing. These materials are actually crosslinked
thermoplastics with the crosslinking occurring either
during the processing or during the annealing cycle. The linear materials are
thermoplastic and are chemically unchanged during molding (except for possible
degradation) and can be reshaped again and again.

As previously discussed, crosslinking can be
initiated by heat, chemical agents, irradiation, or a combination of these.
Theoretically, any linear plastic can be made into a crosslinked
plastic with some modification to the molecule so that the crosslinks
form in orderly positions to maximize properties. It is conceivable that, in
time, all materials could be available in both linear and crosslinked
formulations.

The formulation of a material, crosslinked or linear,
will determine the processes that can be used to successfully shape the
material. Generally, crosslinked materials (thermosets) demonstrate better properties, such as improved
resistance to heat, LESS CREEP, better chemical resistance, etc. than their
linear counterpart: however, they will generally require a more complex process
to produce a part, rod, sheet, or tube.

Some
examples of the various types of materials:Linear Thermoplastics
PVC
Nylon
Acrylic
Polycarbonate
ABS

As discussed in the previous section, the properties of the various families of
plastics vary from one another and the polymers can be modified to alter the
properties within a family of plastics. Another way that the properties of a
given plastic are changed is the addition of items, such as additives,
colorants, fillers, and/or reinforcement.

ADDITIVES
(improve specific properties)
Additives are selected to be compatible with the material and the
process conditions for shaping the material. The improvement of a specific
property of a material by the addition of an additive is usually at the expense
of some other property. The chemist attempts to keep all of the other material
properties as high as possible while achieving the desired improvement in the
specific property, such as improved resistance to burning. Some of the
additives that are used in thermosets and themoplastics are antioxidants to improve high temperature
stability, antistatic agents, biocides, flame retardants, impact modifiers,
friction reducers, foaming agents, fungicides, and ultraviolet stabilizers.

REINFORCEMENTS
(improve strength) Other additives enhance the strength of a
material. Some reinforcing materials are carbon, glass, mica, and aramids. They may be in the form of short fibers,
continuous filaments, mats, spheres, flakes, etc. These reinforcements usually
increase the material's strength at the expense of impact resistance. The use of
reinforcements in plastics permits them to be used at higher temperatures and
loads with greater dimensional stability. The freedom of design, high strength,
and light weight of composite materials are permitting significant advances in
technology in the aerospace and aviation fields. Reinforcements tend to make
stock shapes, such as rods, tubes, slabs, etc., more difficult to machine
because of increased tool wear.

COLORANTS
(change appearance) Another group of additives are colorants that
provide the desired color to the material. The colorants may be organic dyes or
inorganic powder. The colorant chosen must be compatible with the base plastic,
shaping process, and the proposed usages for the finished material. For
example, a colorant must also withstand high temperatures and be weatherable if the material is to be extruded and then used
outdoors. The type of colorant also affects optical properties of transparent
materials, such as acrylics, polycarbonate, and styrene. A colorant can make a
clear material transparent, transluscent, or opaque.

MECHANICAL
PROPERTIES OF PLASTICS

This section will acquaint the reader with the technical terms and concepts
used to describe the properties or performance of a material. It is important
to understand these STANDARDIZED terms since they are used by suppliers and
users to communicate how a material behaves under specific conditions. This
allows comparisons of different materials.

DESIGN
A designer or engineer will often use design equations that work with
metals while a part is being designed. Metals behave like a spring; that is,
the force generated by the spring is proportional to its length. A plot (FIGURE
2) of the force as a function of length is a "straight line."

When a material actually works this way it is called "LINEAR"
behavior. This allows the performance of metals and other materials that work
like a spring to be quite accurately calculated. A problem occurs when the
designer tries to apply these same equations directly to plastics. Plastics DO
NOT BEHAVE LIKE A SPRING (not a straight line), that is they are
"non-linear." Temperature changes the behavior even more. The
equations should be used only with very special input. A material supplier may
have to be consulted for the correct input.

How much load or force will the part be required to
carry? How will the part be loaded? What are the direction and size of the
forces in the part? These are but a few of the questions that a designer tries
to answer before a material is selected.

STRESS
How does one know if a material will be strong enough for a part? If the
loads can be predicted and the part shape is known then the designer can
estimate the worst load per unit of cross-sectional area within the part. Load
per unit area is called "STRESS" (FIGURE 3).

Figure 3

If Force or Load is in pounds and area is in square inches then the units for
stress are pounds per square inch.

STIFFNESS
(Modulus)
Sometimes a designer knows a part can only bend or deflect a certain
amount. If the maximum amount of bending and the shape of the part are known,
then the designer can often predict how STIFF a material must be. The
measurement of the STIFFNESS of a material is called the "MODULUS" or
"MODULUS OF ELASTICITY." The higher the modulus
number, the stiffer the material; and conversely, the lower the number, the
more flexible the material. The Modulus also changes as the temperature
changes. Modulus numbers are also given in pounds per square inch.

PROFESSIONAL PLASTICS, INC.

TYPICAL
TENSILE MODULUS VALUES (PSI)
(at room temperature)

Graphite-epoxy composites

40,000,000

Steel

30,000,000

Aluminum, 1000 series

10,000,000

Epoxy-glass laminates

5,800,000

Polyester-glass reinforced

2,000,000

Nylons, 30% glass reinforced

1,400,000

Acrylics

500,000

Cast epoxy

450,000

Polycarbonate

450,000

Acetal, copolymer

410,000

Polyethylene; high molecular
weight

100,000

STRAIN
The measurement of how much the part bends or changes size under load
compared to the original dimension or shape is called "STRAIN."
Strain applies to small changes in size.

If the change in size is in inches and the original dimension is in inches,
then the units for strain are inch per inch.

STRESS, STRAIN, and MODULUS are related to each other by the following
equation. The modulus or stiffness of a material can be determined when the
material is loaded in different ways, such as tension, compression, shear, flexural(bending) or torsion (twisting). They will be called
TENSILE MODULUS, also know as plain MODULUS, FLEXURAL MODULUS, TORSIONAL
MODULUS, etc.
MODULUS = STRESS/STRAIN
or, in other words
MODULUS = Load /change in shape when loaded. (STIFFNESS)

Choose the type of modulus in the property sheet that most nearly duplicates
what the customer expects the major load to be, tension, bending (flexural). If
the load is unknown, use the lowest moduli value of
the two. These numbers can be used for short-term loading if the load is to be
applied for only a few days at the most.

The stress/strain equation is the equation used by designers to predict how a
part will distort or change size and shape when loaded. Predicting the stress
and strain within an actual part can become very complex. Fortunately, the
material suppliers use tests that are easy to understand.

THE
PERFORMANCE OF A PLASTIC PART IS AFFECTED BY:
WHAT KIND OF LOAD THE PART WILL SEE (Tensile, Impact, Fatigue, etc.)
HOW BIG THE LOAD IS
HOW LONG OR OFTEN THAT LOAD WILL BE APPLIED
HOW HIGH AND/OR LOW A TEMPERATURE THE PART WILL SEE
HOW LONG IT WILL SEE THOSE TEMPERATURES
THE KIND OF ENVIRONMENT THE PART WILL BE USED IN. WILL MOISTURE OR OTHER
CHEMICALS BE PRESENT?

THIS IS WHERE PLASTICS DIFFER IN THEIR BEHAVIOR WHEN COMPARED TO OTHER
MATERIALS, SUCH AS METALS AND CERAMICS. CHOOSING STRESS AND/OR MODULI VALUES
THAT ARE TOO HIGH AND DO NOT ACCOUNT FOR TIME AND TEMPERATURE EFFECTS CAN LEAD
TO FAILURE OF THE PART.

Some
additional terms that are used to describe material
behavior:

YIELD
POINT
The yield point is that point when a material subjected to a load,
tensile, compressive, etc. gives (yields) and will no longer return to its
original length or shape when the load is removed. Some materials break before
reaching a yield point, for example, some glass-filled nylons or die cast
aluminum.

To try to further visualize this property, take a piece of wire and slightly
bend it. It will return to its original shape when released. Continue to bend
and release the wire further and further. Finally the wire will bend and not
return to its original shape. The point at which it stays bent is the
"YIELD POINT." The "yield point" is a very important
concept because a part is usually useless after the material has reached that
point.

TENSILE
STRENGTH
The maximum strength of a material without breaking when the load is
trying to pull it apart is shown in Figure 4. This is the system used by the
suppliers to report tensile properties in their literature, such as strength
and elongation.

Figure 4

A good way to visualize this property is to think of pulling a fresh
marshmallow apart and then pulling a piece of taffy apart. The force or pounds
required to pull the taffy apart would be much greater than required to pull
the marshmallow apart. If that force is measured and the taffy and marshmallow
each had a cross-sectional area of one square inch, then the taffy has the
higher "tensile strength" in terms of pounds per square inch.
Plastics may demonstrate tensile strengths from 1000 psi
(pounds per square inch) to 50,000 psi.

ELONGATION
ELONGATION IS ALWAYS ASSOCIATED WITH TENSILE STRENGTH because it is the
increase in the original length at fracture and expressed as a percentage. An
example would be to pull on a 1 " wide piece of paper that is 4"
long. It tears with no visible elongation or nearly 0% elongation. Now do the
same thing to a 1" x 4" piece of taffy. It will stretch several times
its original 4" length before it fractures. Assume that it is stretched to
a 12" length then (12"/4")(100)= 300%
elongation (FIGURE 5).

Figure 5

COMPRESSIVE STRENGTH
The maximum strength of a material without breaking when the material is
loaded as shown in Figure 6. Check if the material supplier
has the information on compressive strength, since it is not always determined.

This term becomes less meaningful with some of the softer materials. PTFE, for
example, does not fracture. Consequently, the compressive strength continues to
increase as the sample is deforming more and more. A meaningful
"compressive strength" would be the maximum force required to deform
a material prior to reaching the yield point. The compressive term similar to
"elongation" is "compressive deformation," though it is not
a commonly reported term. It is easy to visualize two identical weights (FIGURE
7), one sitting on a 1" cube of fresh marshmallow and the other on a
1" cube of taffy. The marshmallow would be flattened and deformed more.

Figure 6

Figure 7

SHEAR
STRENGTH
The strength of a material when the material is loaded as shown in
Figure 8. The surfaces of the material are being pulled in opposite directions.
Some examples of items that see shear loading are the nail holding a picture on
the wall, the cleats of athletic shoes, and tire tread as a car speeds up or
slows down.

Figure 8

FLEXURAL STRENGTH
The strength of a material when a beam of the material is subjected to
bending as shown in Figure 9. The material in the top of the
beam is in compression (squeezed together), while the bottom of the beam is in
tension (stretched). Somewhere in between the stretching and squeezing there is
a place with no stress and it is called the neutral plane. A simple beam
supported at each end and loaded in the middle is used to determine the
flexural modulus given in properties tables. Skis, a fishing pole, a pole vault
pole, and a diving board are examples of parts needing high flexural strength.

Figure 9

TORSIONAL STRENGTH
The strength of a material when a shape is subjected to a twisting load
as shown in Figure 10. An example of a part with a torsion load is a
screw as it is being screwed in. The drive shaft on a car also requires high torsional strength.

Figure 10

POISSON'S
RATIO
Sometimes a designer will need a value for Poisson's Ratio. This ratio
occurs in some of the more complex stress/strain equations. It sounds
complicated, but it is simply a way of saying how much the taffy (material)
necks down or gets thinner in the middle when it is streched
(FIGURE 11). Its value is most often between .3 to .4 for
plastic materials. Check supplier literature for specific information.

Figure 11

Figures
12 through 16 show the tensile strain curves for different types of materials.
REMEMBER TO THINK OF PULLING ON DIFFERENT KINDS OF TAFFY; THAT IS, SOFT AND
WEAK, HARD AND BRITTLE, ETC.

Figure
12

Figure
13

Figure
14

Figure
15

Figure 16

Figure
17 shows how a plastic material can appear stiffer and stronger if it is pulled
apart faster. An example of rate sensitivity is when we can't pull a string
apart, but we can snap it apart.

Figure 17

Figure
17 also shows how the material is softer and weaker at higher temperatures,
like wax. Plastics are also affected by low temperatures and many become more
brittle as the temperature goes down.

Figure 18

Figure 18 shows the effect of moisture in the atmosphere on the properties of a
material like nylon. The dry material is hard and brittle while the wet
material is softer and tougher. This is like comparing uncooked spaghetti to
cooked spaghetti.

Typical
tensile yield strengths of some materials (psi)

Low alloy hardening steels;
wrought, quenched, and tempered

288,000

High strength low alloy steels;
wrought, as rolled

80,000

Aluminum casting alloys

55,000

Aluminum alloys, 1000 series

24,000

Polyphenylene Sulfide, 40% glass reinforced

21,000

Acetal, copolymer, 25% glass reinforced

18,500

Nylons, general purpose

12,600

Acetal, homopolymer

10,000

Acrylics

10,000

Acetal, copolymer

8,800

ABS/Polycarbonate

8,000

Polypropylene, general purpose

5,200

Polypropylene, high impact

4,300

Creep
Visualize large weights being hung on bars of different materials. All
materials will experience some initial and immediate deformation or stretching
when the load is first applied. As long as the yield point has not been
exceeded, a metal sample which acts like a spring will not stretch any more
regardless of how long the weight is left on. When the weight is removed, the
metal bar will return to its original shape. The length of a "thermoplastic"
bar will continue to slowly increase as long as the load is applied. This is
called CREEP. The amount of creep increases as the load and/or temperature are
increased. Some thermoplastics like nylons will creep more when they have
softened because of the presence of moisture. The "crosslinked"
or "3D net" structure in "thermosets"
resists creep better than thermoplastics. Reinforcements like glass and carbon,
which do not creep, greatly reduce the creep of the composite material when
mixed with a plastic.
Remember the relationship between stress/strain/modul
is:
Modulus = Stress/Strain The initial strain or change in length with the
weight will give a value for the modulus (this is usually the short term value
reported in the property tables for the tensile modulus or flexural modulus).
If the weight (stress) is left on over a period of time, the amount of bending
or elongation continues to increase and the value for the modulus will decrease
with time as shown in Figure 16. This decreasing modulus that is a function of
time (and even temperature) is called the "CREEP MODULUS" or
"APPARENT MODULUS."
THIS IS THE MODULUS THAT THE DESIGNER SHOULD BE USING TO MORE ACCURATELY
PREDICT THE BEHAVIOR OF THE PLASTIC MATERIALS. THE VALUE CHOSEN
FROM THE SUPPLIER'S LITERATURE WILL BE BASED ON THE ESTIMATED TIME THE LOAD
WILL BE APPLIED, THE AMOUNT OF THE LOAD, AND THE TEMPERATURE CONDITIONS PRESENT
WHEN THE LOAD IS TO BE APPLIED.

Figure 19

REMEMBER
THAT CREEP IS AFFECTED BY:
LOAD (STRESS)
TEMPERATURE
LENGTH OF TIME THE LOAD IS APPLIED
OTHER ENVIRONMENTALS, SUCH AS MOISTURE OR CHEMICALS

Since
the STRESS is kept constant, i.e., the weight or load is not changed or
removed, the equation becomes:

Apparent
Modulus x Total Strain = Constant (Stress)

or in other words, if the strain
goes up, then the Apparent Modulus must come down. Since the strain increases
with time and temperature, the Apparent Modulus decreases with time and
temperature.

The data is sometimes presented in supplier literature in terms of Stress
Relaxation. This means that the STRAIN is held constant and the decrease in the
load (stress) is measured over time. This is called "STRESS RELAXATION''.
This information is important for applications, such as gaskets, snap fits,
press fits, and parts joined with screws or bolts. The equation becomes:

Apparent
Modulus / Stress = Constant (Strain)

or in other words, as the stress
goes down because the material moves, then the apparent modulus also goes down.

Sometimes a supplier will recommend a maximum design stress. This has a similar
effect to using the apparent modulus. The recommended design stress for some
acrylic injection molded parts is 500 psi and yet its
tensile strength could be reported to be as much as 10,000 psi
in the property chart. Designers will often look at the 10,000 psi value and cut it in half to be safe; however, it is not
really enough and could lead to failure of the part.

Figure 20

Figure 21

Figure 22 shows the Tensile Elongation of a Material as a function of Time at
Various Stress Levels. Think about pulling a piece of taffy to help visualize
what is happening. The X indicates that the test bar broke. Notice how the
elongation is significantly reduced as the stress level is reduced. A stress
level is finally reached where the creep is nearly negligible.
THESE VALUES WILL BE THE STRESS LEVELS RECOMMENDED AS DESIGN CRITERIA.

Figure 22

Figure 23 shows one of the ways the creep data is often presented in
literature. The time scale is usually over a very long time, hundreds and more
often thousands of hours. Most of the literature will compress the time scale
for ease of reading with the use of a logarithmic scale along that axis.

Figure 23

FATIGUE
STRENGTH
Plastics, as well as other materials, subjected to cyclic loading will
fail at stress levels well below their tensile or compressive strengths. The
combination of tension and compression is the most severe condition. This
information will be presented in S-N Curves or tables. The
S-N stand for Stress-Number of cycles. A PART WILL SURVIVE MORE CYCLES
IF THE STRESS IS REDUCED. The stress can be reduced by reducing the deflection
and/or decreasing the thickness of a part.

Some examples of cyclic loading are a motor valve spring or a washing machine
agitator. With time, parts under cyclic loading will fail; however, properly
designed and tested they will not fail before several million loadings have
been completed.

Figure
24 shows a typical S-N curve.

Figure 24

IMPACT
STRENGTH
Many plastics demonstrate excellent impact strength. Impact strength is
the ability to withstand a suddenly applied load. Toughness is usually used to
describe the material's ability to withstand an impact or sudden deformation
without breaking. No single test has yet been devised that can predict the
impact behavior of a plastic material under the variety of conditions to which
a part can be subjected. Many materials display reduced impact strength as the
temperature is lowered. Thermosets and reinforced
thermoplastics may change less with changes in temperature. Check the supplier
literature for any unusual factors that may affect the impact performance of a
part.

Some of the impact tests commonly used in
supplier literature are:

Figure 25

Izod Test: designed to measure the effect of a sharp
notch on toughness when the test specimen is suddenly impacted.

Tensile
Impact Test: designed to measure the toughness of a small specimen without a
notch when subjected to a sudden tensile stress or load.

Brittleness
Temperature Test: determines ability of the material to continue to absorb
impacts as the temperature is decreased.

Special
tests may need to be devised to more nearly duplicate the actual application.

Figure 26

INFORMATION
PROVIDED BY THESE TESTS WILL AID IN CHOOSING MATERIAL CANDIDATES; HOWEVER, THE
DESIGNER MUST STILL TEST THE ACTUAL PART UNDER CONDITIONS AS NEAR AS POSSIBLE
TO ACTUAL USE CONDITIONS BEFORE BEING CONFIDENT THAT THE MATERIAL SELECTION IS
ADEQUATE.

Figure 27

NOTCH
SENSITIVITY

Some
plastic materials have exceptional impact performance and very good load
carrying capability; however, the performance of a material can be greatly
reduced by having sharp corners on the part. The sharp corners can be part of
the design or from machining operations. A SHARP CORNER IS A GREAT PLACE FOR A
CRACK TO START. The Izod impact strength of a tough
material like polycarbonate is reduced from 20 to 2 as the radius of the notch
is reduced from 0.020"R to 0.005"R respectively.

The
sharp corners not only reduce the impact resistance of a part, but also allow
for a stress concentration to occur and encourage the premature failure of a
load carrying part.

Figure 28

MINIMIZING
SHARP CORNERS MAY MAKE THE MACHINING OPERATION MORE DIFFICULT; HOWEVER, IT MAY
BE CRUCIAL TO THE PART'S SUCCESS.

Edges
of sheet being used in impact applications like glazing must also be finished
to be free of sharp notches. This is a concern with acrylics and even tough
materials like polycarbonate.

THERMAL
PROPERTIES
With a change in temperature, plastics materials tend to change size
considerably more than other materials, such as steel, ceramics, and even
aluminum. A designer must consider these differences in the sizes. In fact, the
shipping environment may expose the part to a much greater temperature
variation than the part will ever see in use. The measure of how much a part
changes size as the temperature changes is called the "THERMAL COEFFICIENT
OF EXPANSION".

COEFFICIENT
OF EXPANSION
The units are usually given in inches per degree Fahrenheit. It is the
change in length (inches) of one inch of a part caused by changing the
temperature one degree.

TYPICAL
COEFFICIENTS OF EXPANSION (in/in/F)

Polyethylene

.000140

Acrylics

.000060

Acetal, copolymer

.000047

Polycarbonate

.000037

Aluminum, 1000 series

.000013

Polycarbonate, 30% glass
reinforced

.000009

Steels

.000008

Glass

.000004

Example: assuming an acrylic material, how much will a 10 inch dimension
change if the temperature changes 40°F?

DEFLECTION
TEMPERATURE UNDER LOAD
In addition to changing size, the strength and modulus of elasticity of
plastic materials tend to decrease as the ambient temperature increases. The
standard test for determining the DEFLECTION TEMPERATURE UNDER LOAD (DTUL) at
66 and 264 psi provides information on the ability of
a material to carry a load at higher temperatures. The 66 psi
means a light load and the 264 psi means a heavy load
on a beam. The temperature of the loaded beam is raised until a certain amount
of deflection is observed. The temperature when that deflection is reached is
called the DTUL. Plastics usually have a higher DTUL at 66 psi
than 264 psi because of the lower load.
Note: The DTUL is sometimes referred to as the Heat Distortion Temperature or
HDT.

Figure 29

TYPICAL DEFLECTION TEMPERATURES,
LOADED TO 264 psi (F)

Silicon materials

850

Nylons, 30% glass reinforced

495

Epoxy, mineral, glass reinforced

400

Acetals, glass reinforced

325

Polycarbonates

295

Nylons, general purpose

220

Acrylics

180

Propylene, general purpose

140

Impact strength is also affected by changes in temperature in most
plastic materials. The changes in strength can be significant, especially as
the temperature is lowered. Check the supplier literature carefully.

THERMAL
CONDUCTIVITY
Plastics are good thermal insulators; that is, heat does not travel
through them easily. We experience this every time we pick up a hot pan by its
plastic handle. The "CONDUCTIVITY" of plastics is 300 to 2500 times
poorer than most metals. This property shows why it takes a long time for a
casting or other molded parts to cool down in the middle. Internal stress can
be set up in a material because of the differences in the cooling rates between
the outside of a part and the core.

EFFECTS
OF THE ENVIRONMENT ON PLASTICS
Environmental factors, such as ambient moisture, chemicals (liquid or
vapor), exposure to sunlight, high temperatures, hot water and/or steam,
bacterial/fungi (underground conditions), and irradiation all tend to attack
plastic materials. Materials may not only change appearance, but have a significant
decrease in properties, such as impact and tensile strength. Again check the
supplier's literature carefully.

Plastic materials do not rust or corrode and many plastics perform
significantly better than metals in corrosive environments. Also understand
that the MORE CHEMICALLY RESISTANT a plastic is, the MORE DIFFICULT it is to
bond to since bonding generally requires some chemical attack.

Chemical resistance is also a critical factor if the part is to be PAINTED. The
solvents in the paint must be compatible with the material to be painted. It is
best to use paints recommended by the material supplier.

Gaskets, "0" rings, or other dissimilar materials that will be in
intimate contact with a plastic over a long period of time MUST not contain chemicals,solvents, or
plasticizers that will leach out and attack the base material. Flexible vinyl
is an example of a material softened by a chemical additive. This vinyl is also
a good example of plasticizer migration (outgassing)
from pieces inside a car and it ends up fogging the windows.

The outgassing of volatiles is accelerated when the
material is exposed to high temperatures and/or vacuum. In critical
applications requiring no outgassing, a material must
be selected that does not contain any plasticizers or other additives that can
outgas. Often, pre-baking the material at a temperature slightly above the
application temperature will drive out most of the volatiles. Check with the
material suppliers. Materials such as polycarbonate, acetals,
nylons, and acrylics have been used in these applications.

ELECTRICAL
PROPERTIES OF PLASTICS
Commercial plastics are generally very good electrical insulators and
offer freedom of design in electrical products. Electrical properties may also
be changed by environmental conditions, such as moisture and/or temperature.

A BASIC CONCEPT TO REMEMBER is that electrons must be exchanged between
molecules for electric current to flow through a material. Plastic molecules
hold on to their electrons and do not permit the electrons to flow easily; thus
plastics are insulators.

The molecules in plastics are also "polar" which means that they will
tend to act like little magnets and align themselves in the presence of a
voltage or field, the same as the needle in a compass trying to point North.

The
electrical properties of plastics are usually described by the following
properties:

VOLUME
RESISTIVITY
The Volume Resistivity is defined as the ratio
between the voltage (Direct Current or DC), which is like the voltage supplied
by a battery, and that portion of current which flows through a specific volume
of the specimen. Units are generally ohm per cubic centimeter.

Visualize putting DC electrodes on opposite faces of a one centimeter (.394
inch) cube of a plastic material. When a voltage is applied, some current will
flow in time as the molecules align themselves (Figure 30).

Figure 30

Ohm's Law tells us that a voltage (volts) divided by the current (amps) is
equal to a resistance (ohms) or V/I = R. When the voltage applied to the cube
is divided by the current, the resistance for 1 cm of the plastic is determined
or ohm per cm.

Generally
plastics are naturally good insulators and have very high resistance. The
Volume Resistivity can change with temperature and
the presence of moisture or humidity.

SURFACE
RESISTIVITY
The Surface Resistivity is the ratio between
the direct voltage (DC) and current along the surface per unit width. Units are
generally ohms.

Again refering to Ohm's Law, The Surface Resistivity is a measure of how much the surface of the
material resists the flow of current.

Figure 31

DIELECTRIC
CONSTANT
The Dielectric Constant is the ratio of the capacitance (AC voltage) of
electrodes with the insulating material between them to the capacitance of the
same electrodes with a vacuum or dry air in between.

The dielectric constant is a measure of how good a material works to separate
the plates in a capacitor. Remember that the molecules are like little magnets
and are trying to realign themselves every time the
voltage (current) changes direction. Some materials do it better than others.

The dielectric constant for a vacuum has a value of 1. Dry air is very nearly
1. All other materials have "dielectric constants" that are greater
than 1. The "dielectric constant" for a plastic material can vary
with the presence of moisture, temperature, and the frequency of the
alternating current (and voltage) across the plates.

The units for frequency are usually "HERTZ (Hz)" which means cycles
per second. 3 kilohertz is the same as 3,000 hz and 3 megahertz is the same as 3,000,000 hz.

DIELECTRIC
STRENGTH
Dielectric Strength is the voltage difference (DC) between two
electrodes at which electrical breakdown occurs and is measured as volts per
mil of thickness. This is an indication of how effective an
"insulator" the material is.

Note:
One mil is another way of saying .001 of an inch, so a piece of plastic film 5
mils thick is .005 inch thick.

The test is similar to that used for "Volume Resistivity"
except the voltage is increased until there is an are
across the plates. This means that the voltage was strong enough to break down
the material and allow a large current to flow through it. Again this property
can be affected by the presence of moisture and temperature. Frequency may also
affect this property when the material is subjected to an Alternating Current.
See Figure 30.

Figure 32

DISSIPATION
FACTOR
The Dissipation Factor (AC) is the tangent of the loss angle of the
insulating material. It can also be described as the ratio of the true in-phase
power to the reactive power, measured with voltage and current 90 degrees out
of phase.

This is an indication of the energy lost within the material trying to realign
the molecules every time the current (voltage) changes direction in alternating
current. The property varies with moisture, temperature, and frequency.

Figure 34

ARC
RESISTANCE
The Arc Resistance is the elapsed time in which the surface of the
material will resist the formation of a continuous conductive path when
subjected to a high-voltage (DC), low-current arc under rigidly controlled
conditions.

Figure 35

EMI/RFI
There is also considerable effort being expended by material suppliers
to try and improve the conductivity of plastics for applications requiring EMI
(electromagnetic interference) and RFI (radio frequency interference)
shielding. This becomes more and more critical as circuitry is getting smaller
and denser. The improvement in conductivity is currently achieved by adding
carbon fibers, metal fiber, and/or metal flakes as a filler in the material or
coating the plastic part with conductive paint.

EMI and RFI are electromagnetic energy that can be emitted by an electronic
product and affect the operation of other electronic equipment near it.
Conversely, energy from the other products could interfere with the operation
of a given product. FCC regulations control the amount of energy that can be
emitted by a product.

Examples of EMI and RFI interference are: when you hear other noise and/or
stations on your car radio; when a CB broadcast is heard on your FM receiver;
when you see snow on your TV set when an appliance is run; warnings in
restaurants that a microwave is being used.

The screen or perforated metal seen in your microwave door is an example of
EMI/RFI shielding. Coaxial cable for your TV antenna is a wire surrounded by a
woven metal shield that is to be grounded. The shield absorbs energy coming in
from outside sources and keeps the signal in the wire pure while preventing
that signal from escaping and interfering with some other electronic product.

Another serious potential problem is the static charge that can be picked up
walking across a room and zap an electronic product.
The charge can often be harmlessly dissipated by correctly grounding the
equipment. The application of an anti-static may also
be used to provide a temporary solution.

OPTICAL/COLORABILITY
PROPERTIES OF PLASTICS
Many plastic materials are transparent and used in optical applications.
Some of these materials are acrylics, styrene, PVC, polycarbonate, ABS, and
Epoxy. The properties measured and presented in the material suppliers
literature are concerned with items, such as the % Haze (cloudiness) in a
material, the transmittance capability (how much light gets through the
material), yellowness index (appearance), and the index of refraction (how much
light is bent as it goes into and out of the material)

Transparent colored materials transmit that portion of the visible spectrum
that allows the eye to see the desired color. Most plastic materials are not
transparent and the color of the base material may limit the selection of
colors available.

WEAR
CHARACTERISTICS OF PLASTICS
Wear characteristics of a material are very difficult to define. It can
mean being resistant to scratching when the part is cleaned. It might mean
being resistant to abrasion when the wind blows sand against it. It might mean
running another part against it. It might mean being able to maintain its
appearance after considerable handling.

A material like glass may be very resistant to scratching yet can be readily
abraded by sand blasting, as evidenced by the pits in a windshield. Conversely,
another material like acrylic is easily scratched when wiped and yet is much
more resistant than glass to abrasion from sand blasting. It is usually best to
devise a test that will duplicate actual use conditions to accurately determine
a material's suitability for an application.

Many plastics are specifically formulated for running against surfaces. The
base polymer may exhibit self-lubricating properties. Additives such as TFE,
silicone oil, molybdenumdisulfide, and carbon are
used to further enhance the bearing capabilities of some materials. Materials
have their bearing properties even further enhanced by the addition of
additives, such as TFE.

MACHINABILITY
Plastic stock shapes may be easily machined; however, the tool geometry
and speed must be adjusted for optimum performance with a specific material.
The tolerances for machining plastics usually should be larger than applied to
metals. The tolerances must be larger because of thermal expansion and the
shape changing from the relaxation of internal stresses within the material. In
critical applications, it may be necessary to premachine
the part slightly oversize and STRESS RELIEVE or ANNEAL the part before taking
the final cuts.

Annealing is the baking of a material, without melting or distorting the part,
for a time to relax the internal stresses. The internal stresses are usually
caused by uneven cooling, that is the outside of the part cools much faster
than the inside when the blank is made. This uneven cooling can also cause
variations in the properties from the outside to the inside.

The poor thermal conductivity of plastics requires that care is taken to
prevent the area being machined from getting too hot. The type of tool, depth
of cut, rate of feed, and coolant flow may have to be adjusted. If a coolant is
used, MAKE SURE IT DOES NOT CHEMICALLY ATTACK THE
PLASTIC BLANK.

Check
the supplier literature for specific recommendations on the types of tools,
speeds, etc., to be used with a particular material.

TOLERANCES
Many designers will ARBITRARILY put a +/-.005
tolerance on a part if it is to be machined. Quiz the designer if the
tolerances can't be increased. Remember that a piece of paper is about .003
inch thick, +/- .03 is equal to 1/16 of an inch, and +/- .06 equals 1/8 of an
inch. Look at a ruler to visualize the size of the tolerance and think about
the tools available to make the cut. Work with the designer to specify the tolerances
really needed to make his part work and that can really be produced with the
equipment available.

PROCESSING
Plastics are changed into useful shapes by using many different
processes. The processes that are used to mold or shape thermoplastics
basically soften the plastic material so it can be injected into a mold, flowed
through a die, formed in or over a mold, etc. The processes usually allow any
scrap parts or material to be ground up and reused. Some of the more common
processes are injection molding, extrusion, blow molding, rotational molding, calendering, thermoforming (which includes vacuum forming),
and casting.

INJECTION
MOLDING
"Injection Molding" is used to make three dimensional shapes
with great detail. The material is placed in the hopper of an injection molding
machine where it is fed into a chamber to be melted. The melting is achieved by
conducting heat into the material in a "Plunger" machine, while the
material is primarily heated by shearing or mechanically working the material
in a "Screw" machine. Several shots of material are being heated and
held in the injection unit. The maximum volume of material a machine can inject
in a single shot determines its shot capacity. The capacity is given in ounces
of a material.

Once melted the material is forced, under pressure, into the mold where it
conforms to the shape of the cavity. The mold is temperature controlled,
usually by circulating temperature controlled water through it. Once the part
is cooled, the mold is opened and the part removed. The mold is then closed and
ready for the next shot. The mold is clamped shut while the material is being
injected in to the cavity since the cavity pressure may be as much as 5,000 psi. The clamp is sized by the "Tonnage" it
holds. Injection molding machines will be referred to by its shot size in
ounces and its tons of clamping ability. An example would be a 6 oz, 80 Ton
machine.

The molds are most often made out of hardened steel and carefully finished.
They may also be made out of prehard steel, aluminum,
epoxy, etc. The type of mold material selected depends on the number of parts
to be made and the plastic material to be used. Parts are often machined to
test the shape and function of a part before a mold is built.

EXTRUSION
"Extrusion" is like squeezing toothpaste out of its tube. The
process produces continuous two dimensional shapes like sheet, pipe, film,
tubing, gasketing, etc. The material is fed into the
extruder where it is melted and pumped out of the extrusion die. The die and
the take-off line shape the material as it cools and control the final
dimensions of the cross-section of the shape. The equipment is designed and
controlled to produce melted plastic at a very uniform temperature and pressure
which control the size and quality of the extruded product.

The extrusion process is also used with a system of molds and called "Blow
Molding." This is how bottles, such as the gallon milk bottle, are
produced.

THERMOFORMING
An extruded or cast sheet can be heated, draped over a mold, and allowed
to cool to produce a part. This process is called thermoforming. The material
can be made to better conform to the shape of a mold by using a vacuum to pull
the material down. A bubble or shape can also be blown up with air pressure.
These are but two of the techniques that can be used to push the material into
some desired shape. They basically require that the material be softened so a
low force can be applied to shape the part. Signs, skylights, bubble packaging,
boat and motorcycle windshields are some examples of parts made using this
process.

CALENDERINGCalendering is a process that usually uses
four heated rolls rotating at slightly different speeds. Again the material is
fed into the rolls, heated and melted, and then shaped in sheet or film. PVC is
the most commonly calendered material.

CASTING
Acrylic and nylons can also be cast. Just as the name implies, the
material in a liquid form is poured into a mold and hardened. The process
requires considerable process control to obtain high quality parts. Tubing,
rods, sheets, and slabs are often made this way.

THERMOSETSThermosets must use a process that allows the
material to flow to the desired shape and then become crosslinked
and rigid. The material cannot be remelted or reused
after crosslinking occurs. Some of the processes
commonly used to process thermoset materials are
injection molding, transfer molding, compression molding, hand (or spray)
lay-up, lamination, and filament winding.

The injection molding of thermosets is similar to the
injection molding of thermoplastics except the material is kept cool until it
is pushed into the heated mold where it is crosslinked.
The mold is then opened and the hot, but rigid, part is removed.

TRANSFER
MOLDING
In transfer molding, only enough material for
one shot is placed in a separate chamber or pot. The material is then pushed
from the pot into the hot mold and crosslinked. All
of the "cured" material is removed from the machine and another
charge loaded for the next shot.

COMPRESSION
MOLDING
A single charge of material is placed directly into the cavity of the
heated mold. The material flows and fills the cavity as the mold closes. The
mold is kept closed until the material crosslinks.
All of the cured material is removed from the mold prior to recharging the
cavity.

HAND
(OR SPRAY) LAY-UP
Hand lay-up is used to produce products, such as fiberglass boats and
camper shells. The plastic resin, usually a polyester,
is rolled or sprayed with glass reinforcement into a mold. A catalyst is added
to the material to cause the material to crosslink or harden at room
temperature. This process lends itself to making large and strong parts.

LAMINATINGThermosets are also used in making laminates.
The materials to be laminated are stacked in a press, clamped, and heated. Some
examples of laminates using thermosets are plywood
(the adhesive), electronic circuit boards, cloth reinforced phenolic
sheet, and counter top laminates.

FILAMENT
WINDING
Filament winding is an automated version of the hand lay-up process.
Reinforcing filaments are covered with a resin and then wound over a mandrel.
The number of layers and orientation can be varied depending on the load that
the part is to carry. A strong thin hollow part is left after the mandrel is
removed. Storage tanks and street lighting poles are some examples of filament
wound parts.

There are many other processes, too numerous to mention in this text. It is
suggested that the reader obtain other literature that can provide more
information, in greater depth, on the various processes.

MATERIAL
SELECTION
The selection of a material for an application is a very difficult task.
Usually one is only able to narrow the selection down to two or three
candidates and the final selection is then determined by testing. SOMETIMES THE
SELECTION IS DETERMINED BY THE BEST MATERIAL IMMEDIATELY AVAILABLE SO THE
SCHEDULE CAN BE MET OR THE LEAST EXPENSIVE MATERIAL. This DOES NOT always lead
to a successful application or a satisfied customer.

As a plastic materials professional, one must be alert to those applications
that are not correct for plastics. Sometimes a designer or customer becomes
enamored with using a plastic without understanding the properties of plastics
and if a plastic material is even suitable for the application. One must also
be careful of a design that is worked in aluminum or steel and is to be
converted to plastic. A metal part may not work in plastic. THIS IS WHERE IT IS
IMPORTANT TO UNDERSTAND WHAT THE CUSTOMER EXPECTS THE PART TO DO. A material
supplier may have to be consulted before the customer can be given a
suggestion.

The first and most important step in selecting a material from the broad
spectrum of materials (steel, aluminum, brass, polycarbonate, acrylic, nylon,
etc.) is to carefully define the requirements of the application. The second
step is to try and match those requirements to the properties of the available
materials.

It may be necessary to ask some or all of the following questions to define the
application. One will develop expertise in how to ask questions with
experience. The more completely the application is defined, the better the
chance of selecting the best material for the job.

WHAT
LOAD WILL THE PART HAVE TO CARRY?
Will the design carry high loads? What will the highest load be? What is
the maximum stress in the part?What
kind of stress is it (tensile, flexural, etc.)? How long will the load be applied? What is the projected life of the part or
design?

Note:
Thermosets often perform well under high continuous
loads. Reinforced thermoplastics, such as a thermoplastic
polyester, may also perform satisfactorily.

WILL
THE PART HAVE TO WITHSTAND IMPACT?
Will the part be subjected to impact? Which impact test/data more nearly
duplicates the projected application?

WILL
THE PART SEE CYCLIC LOADING (FATIGUE)?
Will the part be subjected to a variable load? Is the load alternating
compressive/ tensile? What will the stress levels be? What is the thickness of
the part being flexed?How
much will the part be deflected?

Note:
Materials like acetal and nylon are generally good
candidates for cyclic loading.

WHAT
TEMPERATURES WILL THE PART SEE AND FOR HOW LONG?
What is the maximum temperature the material will see in use? What is
the minimum temperature the material will see in use? How long will the
material be at these temperatures? Will the material have to withstand impact
at the low temperature?

Note:
The temperature extremes could occur during shipping.

WILL
THE MATERIAL BE EXPOSED TO CHEMICALS OR MOISTURE?
Will the material be exposed to normal relative humidity? Will the
material be submerged in water? If so, at what temperature?
Will the material be exposed to steam? Will the material be painted? Will the
material be submerged or wiped with solvents or other chemicals? If so, which
ones? Will the material be exposed to chemical or solvent vapors? If so, which
ones? Will the material be exposed to other materials that can outgas or leach
detrimental materials, such as plasticizers?

WILL
THE MATERIAL BE USED IN AN ELECTRICAL DESIGN? What voltages will the part be
exposed to? Alternating (AC) or direct (DC) current? If AC, what frequencies?
Where will the voltage be applied (opposite side of the material, on one
surface of the material, etc.).

Note:
Enough carbon reinforcement can make a plastic conductive.

WILL
THE MATERIAL BE USED AS A BEARING OR NEED TO RESIST WEAR?
Will the material be expected to perform as a bearing? If so, what will
the load, shaft diameter, shaft material, shaft finish, and rpm be? What wear or abrasion condition will the material see?

Note: Materials with friction reducers added, such as TFE, molybdenumdisulfide,
or graphite, generally exhibit less wear in rubbing applications.

DOES
THE PART HAVE TO RETAIN ITS DIMENSIONAL SHAPE?

What
kind of dimensional stability is required?

Note: An application requiring a very high level of dimensional stability may
not be suitable for plastic materials. Remember that the plastic materials move
more with changes in temperature than do metals.

The
most stable plastics are reinforced with glass, minerals, etc..

WILL
THE MATERIAL HAVE TO STRETCH OR BEND A LOT?
Are rubberlike properties needed?Does the material have to stretch?

Note: A flexible material like flexible vinyls,
urethanes, rubber, or a thermoplastic elastomer may
be used.

WILL
THE PART HAVE TO MEET ANY REGULATORY REQUIREMENTS?
Is an Underwriter's Laboratories (UL) listed material required? If so,
which rating? Is a UL yellow card required? Is a low
smoke generating material required (FAA)? Is an FDA approved material required
(taste/odor)?

Note: Make sure the supplier has approval from the desired agency and not just
its own lab. The customer may require proof of approval.

DOES
THE MATERIAL OR FILM HAVE TO PREVENT CERTAIN GASES OR LIQUIDS FROM PASSING
THROUGH?

Does
the material have to be impermeable to gases or liquids? If so, which ones?

Note:
This is important for packaging foods and some medical applications.

WILL
THE PART BE EXPOSED TO ANY RADIATION?
Will the material be exposed to radiation? If so, how
much and how long?

Note: This requirement could occur for military, utility (atomic power plants),
or medical applications.

DOES
THE MATERIAL HAVE TO HAVE A SPECIAL COLOR AND/OR APPEARANCE?
What color material is desired? Does it have to match anything else? Is
a textured surface needed?

Note: Direct customers toward the colors that are
readily available from the suppliers. Special colors can be more costly, expecially in small quantities.

DOES
THE PART HAVE ANY OPTICAL REQUIREMENTS?
Does the material need to be transparent? Does the material need to
transmit any particular wavelengths? If so, which ones?

Note: Acrylics and polycarbonates have excellent optical properties.

WILL
THE PART BE USED OUTDOORS?
Note: Acrylics have excellent weatherability.

CAN
ANY VOLATILES BE GIVEN OFF BY THE MATERIAL?
Note: This is often referred to as outgassing.